1. Field of the Invention
The present invention relates generally to signal measurement systems and, more particularly, to scaling displayed waveforms in signal measurement systems.
2. Related Art
Conventional signal measurement systems such as digital oscilloscopes sample, record and display time-varying analog signals. Samples of an input signal are taken and quantized, and the resultant digital representations are stored in a waveform memory under the control of a sampling clock. The acquired data may subsequently be read out as locations in memory are sequentially addressed by a clock signal to provide digital data which can be converted to a time-varying output signal for a waveform display. The sampling clock may be operated at one of several selectable rates depending upon the frequency content of the input signal. The selection of the portion of the analog input signal which is sampled and stored is determined by appropriate triggering circuitry to enable the operator to display the desired portion of the waveform.
There are many types of display elements which can be presented in signal measurement systems in general and test and measurement instruments in particular. For example, in addition to the waveforms representing the signals currently received at the channel inputs, waveforms referred to as function waveforms may also be displayed. Function waveforms are waveforms created by processing the signal waveforms such as performing arithmetic manipulations or combining multiple input signal waveforms in some predetermined manner. The resulting waveforms are placed in a display memory for subsequent retrieval and display. In addition, memory waveforms may also be displayed. Memory waveforms are waveforms which have been stored in memory for some predetermined time for later display. In addition to the above waveforms, other display elements such as markers indicators, trigger indicators, etc. are typically displayed.
Conventional test and measurement systems typically provide a display grid on which the display elements are presented. The display grid divides the coordinate axes into a series of divisions. Waveforms are displayed on the display grid and are scaled vertically and horizontally to facilitate analysis. Typically, the horizontal scale represents sweep speed and is in units of seconds per division. The vertical scale represents signal amplitude and is in volts per division. The center of the horizontal axis represents the delay or horizontal position of the displayed waveform and is referred herein to as horizontal offset. The center of the vertical axis represents the voltage offset of the displayed waveform and is referred to as vertical offset. The adjustment of these parameters is generally referred to signal scaling or waveform scaling and the above parameters are referred to as scaling parameters. Thus there are four scaling parameters which are controlled by the user to capture a desired portion of a waveform and to achieve a desired relative display of multiple waveforms: horizontal scale, horizontal offset, vertical scale and vertical offset.
Conventional test and measurement systems typically have one or more dials or knobs to control waveform scaling. Some conventional systems provide four knobs, one for each scaling parameter, for each input channel in the instrument. In test and measurement systems which have a single time base generator, there are often vertical scale and offset knobs dedicated to each channel and a single common knob dedicated to horizontal scale and offset. In other types of conventional test and measurement systems there is just one set of knobs for vertical scale and offset control. In this type of system, the user must first select which waveform is to be modified in accordance with the knob adjustments. Other conventional signal scaling systems have a single general purpose knob that requires the user to assign to the knob both, the scaling parameter to be changed and the waveform to be modified. The assignment of scaling parameters and/or waveforms to a knob may be achieved through activation of hardware and software switches.
There are a number of drawbacks to these conventional systems. First, there are numerous control actions required to assign parameters and waveforms to individual knobs and numerous adjustments that have to be made to achieve a desired scaling. This yields a complicated user interface that is difficult to learn and operate. In addition, these scaling parameters are adjusted independently; the user can typically modify only a single scaling parameter of a single waveform at a given time. As a result, complete scaling operations require a minimum of several control operations. In addition, it is not uncommon to return to a particular scaling parameter/waveform to make further modifications after other scaling parameters have been modified. Thus, because multiple controls are involved, these manipulations can be iterative in nature, further complicating the use of the instrument. In addition, such iterative control operations often take considerable time, reducing the utilization of the instrument for signal analysis and measurement.
Furthermore, the relationship between the numeric values associated with the scaling control operation and the resulting effect on the displayed waveform is unclear. Users often cannot anticipate the extent to which a waveform changes in size and/or position in response to a specific change in a numeric value.
What is needed, therefore, is a simple, uncomplicated means for enabling a user to perform scaling operations on displayed waveforms quickly and easily without having to perform a large number of control steps and operations and which enables the user to anticipate the resulting effect on the displayed waveforms.